1. Technical Field
This invention pertains generally to the field of network processing, more particularly to maintaining cell integrity during switch-over between active and redundant switch fabrics planes. This invention further pertains to a method of segmentation and reassembly of packets that are transmitted through switch fabrics that results in high cell integrity.
2. Description of the Prior Art
Packet switching has long been used as an alternative to conventional circuit switches. One reason is because conventional circuit switches are far too expensive for intensive or interactive communications. Packet switching involves the transmission of data packets across a shared network. These data packets are also called datagrams. Data packets are individually addressed so that packet switches can route each packet over the most appropriate and available circuit. This allows each packet to survive independently. Each packet may represent an individual set of data, or a larger set of user data can be fragmented into multiple packets. In either case, data propagates through the network using independent packets.
Because the data packets traverse the network independent of each other, user data packets can arrive out of sequence. This results in random communication errors and the subsequent need for retransmission of lost or corrupted data packets. Packet switched networks exhibit unpredictable and variable delivery times. Because of this, packet switching has traditionally been considered unviable for stream-oriented communications such as real-time voice or video.
In the known art, packet-switched networks perform the process of error detection and correction at each of the packet switches. This improves the integrity of data transmission. These error correction processes demand extensive computational resources, resulting in added cost and complexity in the packet switches. Additionally, the process is time-consuming because each packet must be checked for errors prior to being forwarded to the next node. The time consumed during the error-checking process imposes some level of latency on each packet.
An additional disadvantage in traditional systems is that cells can get lost during switchover if a switch fabric is buffered and queued. Using a non-buffered switch fabric eliminates this possibility, but a non-buffered switch fabric exhibits lower overall throughput. In the basic buffered and queued switch fabric, switchover will not be lossless. All cells in the active switch fabric will have to be discarded once it transitions to an inactive state. The cells in the standby switch fabric will have to be discarded, too, since it cannot be guaranteed that both planes of the switch fabric operate in a synchronous manner at the micro-cycle level. Therefore, arrival of fragmented and otherwise invalid cells is very likely. These errors can be detected and corrected in higher layers of the network protocol; so lost cells don't pose much of a problem to data communications integrity. In telecommunication systems, however, the situation is different. Here time division multiplexed (TDM) traffic is predominant, and there is no higher layer function verifying this traffic in a plesiochronous digital hierarchy (PDA). This can become especially critical if system signaling, such as ISDN/ISUP, POTS or CCS#7, is affected.